Magnetic refrigeration technology has been known for more than twenty years and the advantages it provides with regard to ecology and sustainable development are widely acknowledged. Its limits in terms of its useful calorific output and its efficiency are also well known. Consequently, all the research undertaken in this field relates to improving the performance of such a generator, by adjusting the various parameters, such as the magnetization power, the performance of the magnetocaloric element, the heat exchange surface between the heat transfer fluid and the magnetocaloric elements, the performance of the heat exchangers, etc.
The choice of the magnetocaloric materials is determining and has a direct effect on the performance of a magnetocaloric heat generator. To increase the performance, a solution consists in associating several magnetocaloric materials having different Curie temperatures in order to increase the temperature gradient between the ends of this assembly.
So, heat generators are known, comprising at least one thermal module M such as the one represented in FIGS. 1A and 1B and which comprises magnetocaloric materials MC arranged and aligned side by side, and circulation means for the heat transfer fluid, such as pistons P, intended for moving the heat transfer fluid with a reciprocating movement through the whole of the magnetocaloric materials MC, to both ends of these, between the cold side F and the hot side C of the assembly of magnetocaloric materials MC, and in synchronization with the variation of a magnetic field. As shown in FIGS. 1A and 1B, these pistons P are arranged at both ends of the assembly of magnetocaloric materials MC and move alternately in one direction and in the other, FIGS. 1A and 1B show the pistons in their both extreme positions.
It appears in FIGS. 1A and 1B that the fluid moves either in one direction, towards the hot end C (the direction of movement of the heat transfer fluid is represented by the dotted arrows, see FIG. 1A) when the magnetocaloric materials undergo a heating cycle, or in the other direction, towards the cold end F (the direction of movement of the heat transfer fluid is represented by the solid arrows, see FIG. 1B) when the magnetocaloric materials undergo a cooling cycle.
This thermal module M has a disadvantage linked with the fact that, to reach a temperature gradient, a heat transfer fluid must be circulated through the whole of the materials. The use of several magnetocaloric elements MC leads to an increase of the length of the material across which the heat transfer fluid flows. Thus, in order not to reduce the number of cycles (a cycle being defined by a heating and a cooling of the magnetocaloric element), it is necessary to increase the speed of the heat transfer fluid. But the increase of the speed leads to an increase of the pressure, which increases the head losses and reduces the efficiency of the heat exchange between the heat transfer fluid and the magnetocaloric elements, which in turn leads to a decrease of the calorific output of the magnetocaloric generator.
It is also known that, to increase the calorific output of a magnetocaloric generator, it is possible to increase the number of cycles. But this leads to an increase of the speed and also to the above-mentioned disadvantages.
A generator comprising a thermal module M as shown in FIGS. 1A and 1B requires a significant amount of operating time to reach an utilizable temperature gradient between both ends, due to the multiplicity of the materials used.